The production of melamine starting from urea is a method that has been known for quite some time, a distinction being drawn between two types of processes: The noncatalytic high-pressure process and the catalytic low-pressure process. The high-pressure process requires pressures of at least 8 MPa, while the catalytic low-pressure process is carried out in a fluidized bed at a pressure of no more than 1 MPa and temperatures of at least 380 to 410° C. The carrier gas used for the low-pressure method comprises either ammonia or a mixture of carbon dioxide and ammonia, the resulting melamine being present in gaseous form after the reaction.

The reaction of urea to form melamine is endothermic, so that the system must be provided with large amounts of external heat.
The yield of melamine with this method in relation to the quantity of urea used is 90 to 95%. In the literature, the three most frequently used low-pressure methods are known as BASF, Chemie Linz and Stamicarbon processes.
The BASF process is a single-stage reaction method (FIG. 1), where liquid urea is reacted in a fluidized bed at a temperature of 395 to 400° C. under nearly atmospheric pressure. In addition to melamine, the resulting reaction gas includes traces of by-products such as melem and melam as well as reaction gas comprising ammonia and carbon dioxide. The reaction gas mixture that is obtained is then cooled, the removed catalyst and the crystallized by-products are separated out and the reaction gas comprising the melamine is fed to a crystallizer. In the crystallizer, the melamine-containing gas is cooled with reaction gas to lower the temperature of the melamine-containing gas to a range of 190 to 210° C. Under these conditions, melamine desublimes approximately 98% from the reaction gas. After separating out the melamine, the remaining gas (recycle gas) is pumped to a urea-washing station by means of a recycle gas blower, where it is cooled and washed in direct contact with the liquid urea. The temperature of the quenching gas is ˜138° C., so that it is necessary to admix 2.5 to 3.5 kg of quenching gas per kilogram of melamine-containing gas to get a temperature of 190 to 220° C. in the crystallization apparatus.
The production of melamine is a fully developed process that has been known for a long time. For example, German unexamined patent application DE 33 02 833 [U.S. Pat. No. 3,321,603] describes a method of the catalytic production of melamine by means of thermal conversion of urea. In this method, the synthesis gases from which the melamine has already been deposited are treated with a urea melt, wherein the treatment is performed in a scrubber.
The problems with the known process, however, are that due to the endothermic chemical reaction large amounts of heat are required to maintain the fluidized-bed reactor at the required reaction temperature of 395 to 400° C., and additionally that cooling of the melamine-containing reaction gas exiting the fluidized-bed reactor is essential to precipitate undesirable by-products, such as melem or melam. Cooling the gas comprising melamine creates some difficulty. The start of the desublimation process is dependent on the pressure and content of melamine in the gas exiting the reactor. Once the partial pressure reaches the is saturation vapor pressure of melamine, the melamine will crystallize. It is therefore essential that the cooler downstream of the reactor does not cool the gas too much, which would bring about the desublimation process already there. It must also be ensured that in the cooler itself no locations (meaning pipe walls) are below the desublimation temperature because otherwise melamine would crystallize. These melamine adhesions result in drastically shortened plant operating life, undesirable production failure and an increased need for maintenance. To prevent this type of adhesion, heat is removed with the help of a special heat transmission medium (thermal oil) that operates at a temperature at which the critical pipe wall temperature of the cooler does not drop below the melamine desublimation temperature. The thermal oils are higher molecular hydrocarbons (for example phenyls, derivatives thereof and mixtures thereof), which upon release may have severe impact on humans and the environment as well as objects (toxic, flammable). The thermal oil in turn must be treated in a downstream cooler or condenser.